This invention relates to an improvement in making polyethylene terephthalate useful in molding containers such as bottles. More specifically, this invention relates to an improved process for producing polyethylene terephthalate which is good for making beverage bottles.
The hazards of using glass containers for beverages such as beer or carbonated beverages are well known. Dangerous breakage often takes place due to the internal pressure exerted by the pressurized gas in the bottles as well as by dropping or otherwise impacting the bottles, not only in the course of production and distribution of the bottled product, but also in handling of the bottled product by consumers.
In recent years, plastic has been used to replace glass containers, especially in the carbonated beverage industry. Besides avoiding the hazards of breakage, plastic bottles also have the advantage of being much lighter than glass. Furthermore, less energy is required to make and transport plastic bottles.
Polyethylene terephthalate (hereinafter PET) is a polymer which is particularly well suited for such applications. In addition to providing safety and weight benefits, PET has the further advantage of chemical inertness and optical clarity. PET's superior clarity yields extremely clear bottles, providing superior aesthetics in comparison to other polymers, and replicating clear glass most closely.
PET may be prepared, as is well known, by the ester interchange of dimethyl terephthalate with ethylene glycol or by the direct esterification of ethylene glycol and terephthalic acid, followed by condensation polymerization (hereinafter "polycondensation") in the presence of a catalyst such as antimony trioxide, e.g., at a temperature of about 275.degree.-300.degree. C. and at an absolute pressure of about 1 millimeter of mercury. The PET product may then be extruded and pelletized to produce polymer chip.
The PET chip is then subjected to solid state polymerization in order to increase the polymer's intrinsic viscosity and to remove acetaldehyde produced in the pellets during manufacture. It is widely known in the art that the intrinsic viscosity of PET may be increased by solid state polymerization conducted in either air or an inert gas. In this connection, see e.g. U.S. Pat. Nos. 4,223,128 and 4,064,112, respectively. Polyester suitable for use in making beverage containers have a desirably high intrinsic viscosity, i.e., above about 0.60 deciliters per gram as calculated from measurements made on an 8% solution in o-chlorophenol at 25.degree. C.
It is also widely known in the art that solid state polymerization may be used to eliminate acetaldehyde contained in the polyester chip. Acetaldehyde is one of several byproducts created during polycondensation, and remains in the polymer chip after polymerization is completed. Acetaldehyde must be minimized because it can introduce flavor to the drinks contained in the bottles, which is highly undesirable. The acetaldehyde trapped in the chip during the polycondensation reaction is widely known as "free" acetaldehyde. In addition to free acetaldehyde, acetaldehyde may also be formed by the further reaction or decomposition of PET and other byproducts ("acetaldehyde precursors") trapped in the polymer chip during polymer manufacture. Acetaldehyde produced from the reaction of trapped acetaldehyde precursors or which is formed during degradation occurring during the molding process is referred to as regenerated acetaldehyde. Reactions which produce regenerated acetaldehyde are induced when the polymer is subjected to high temperatures, such as those temperatures utilized in the injection molding process.
The elimination of acetaldehyde in beverage containers is important because the presence of acetaldehyde affects the taste of the beverages contained within the PET bottle. Stringent standards are imposed by molders producing beverage bottles, especially those molders who supply bottles for mineral water and cola flavored beverages. In particular, the amount of acetaldehyde present in bottles suitable for mineral water is extremely low.
Accordingly, a need exists for a commercially acceptable PET resin which does not generate significant amounts of acetaldehyde when heated for molding into a container or bottle.
In addition to the requirements above, PET resin suitable for use as a clear glass replacement must provide adequate aesthetic characteristics. The PET resin must produce a bottle which does not exhibit a high level of color in order to replicate the appearance of clear glass. In particular, in order to mimic clear glass, commercially produced PET resin is preferably characterized by a yellowness range of -30 to 6, most preferably by a range of -10 to 4 and is further characterized by a brightness value preferably in the range of 60 to 90 and most preferably in the range of 65 to 85.